In recent years, there have been concerns over depletion of petroleum resources, and problems of waste disposal and the like from the viewpoint of natural environmental preservation and the like. In particular, molded articles and processed articles of common general-purpose polymer materials, when landfilled as wastes, may remain semi-permanently as foreign matters because of their low microbial degradability and disintegratability. Additionally, there have been problems, such as possibility of elution of additives, such as a plasticizer, which are added in order to extend the process stability and product life of polymers, to contaminate the environment and the like. Also, in the case of incineration as wastes, the high combustion heat quantity, which may damage furnaces, and flue gas and exhaust gas generated by combustion, which may cause environmental contamination, are perceived as problems.
Against these backgrounds, biodegradable polymers that are degradable in the natural environment and their molded articles are required, and studies have been actively conducted on naturally degradable resins such as aliphatic polyesters. Of aliphatic polyesters, in particular, lactic acid-based polymers have a sufficiently high melting point of, for example, 170 to 180° C. and also excel in transparency. Thus, lactic acid-based polymers are greatly promising as packaging materials and molded articles and the like for which their transparency is exploited, and some of the materials and articles have been commercialized. Also, lactic acid-based polymers have an easily hydrolyzable characteristic in the presence of water while being robust. Even if such polymers are discarded in the environment, their influence on the environment is reduced as compared with that of conventional general-purpose resins. Also, in the case of indwelling as a medical material in the living body, lactic acid-based polymers have biodegradability and bioabsorbability, provide low-toxic degradation products, and are friendly to the living body because of their biodegradability and bioabsorbability in the living body. Having such excellent properties, lactic acid-based polymers are promising as drug delivery systems (DDS) and medical materials such as bone fixing materials and stents, some of which have been commercialized.
An aliphatic polyester is obtained, for example, by ring-opening polymerization of an aliphatic cyclic ester in the presence of a polymerization initiator and a catalyst (see, for example, Patent Document 1). A lactic acid-based polymer is obtained by a method such as a method for ring-opening polymerizing a lactide, which is a cyclic ester, singly or in combination with another monomer having biodegradability at a temperature at or above the melting point of the polymer, and a method for condensation-polymerizing lactic acid, which is a hydroxycarboxylic acid, singly or in combination with another monomer having biodegradability. Additionally, a method combined with solid-phase polymerization and the like are known.
Generally, in molding processing of a lactic acid-based polymer, since the polymer is heated at or above the melting point of the polymer, it is known that hydrolysis, depolymerization, and cyclic oligomerization as well as intermolecular and intramolecular transesterification and the like may occur depending on the purification degree of the polymer or the like. In particular, when the polymer before molding processing is insufficiently dried, the polymer may hydrolyze in processing, and the resulting molded article may not be able to achieve sufficient physical properties. Furthermore, the polymerization catalyst remaining in the lactic acid-based polymer serves as a depolymerization catalyst to degrade the polymer into monomers, and thus may reduce the molding processability and degrade the physical properties of the molded article.
Moreover, in molding processing of a lactic acid-based polymer, if a large amount of remaining catalyst and residual monomer exists, coloring during the molding processing may be significantly facilitated to markedly impair the appearance of the resulting molded article, and additionally, the stability such as heat stability may be reduced. Usually, to reduce such influences, means are taken to add additives such as a heat stabilizer, a processing stabilizer, an antioxidant, and a catalyst quencher to the polymer, or demonomerization, catalyst removal or the like is performed in the final step of polymer production. Meanwhile, in application of lactic acid-based polymers in the medical field, such additives currently cannot be added, from the viewpoint of their low toxicity and the like. Accordingly, since measures are taken to increase the molecular weight of the material lactic acid-based polymer and the like in consideration of a decrease in the molecular weight during molding processing, the molding processability may conversely be deteriorated or variation in the quality and coloring may be caused. In particular, bone fixing devices and the like, which are desired to have high strength and maintain the strength for a predetermined period by controlling the hydrolysis rate, require a molecular weight higher than that desired for general purposes. Thus, currently, lactic acid-based polymers as a raw material are purified via complex processes comprising washing with a solvent and the like to improve heat stability and reduce coloring.
In view of the current situations described above, methods are studied for purifying a lactic acid-based polymer to improve the stability such as heat stability. For example, in Patent Document 2, a technique is described for mixing a lactic acid-based polymer, after treatment with hydrogen chloride gas in an organic solvent, with a precipitant to allow the polymer to deposit. In accordance with this method, the catalyst in the polymer can be converted into the form of chloride followed by being removed to improve the stability. However, enormous efforts and costs are required because a solvent is necessary, and specialized equipment is also necessary for handling hydrogen chloride. Moreover, no coloring of the polymer is mentioned in any way, and the hue of the resulting polymer and the like are unknown.
In Patent Document 3, a method for purifying and stabilizing solid particles of a high-molecular-weight polylactide is described, wherein the particles are brought into contact with methanol followed by acetone to extract unreacted monomers and residual catalyst. However, in Patent Document 3, since a solvent is required, enormous efforts and costs become necessary. In addition, although removal of the residual catalyst is mentioned, the coloring of the resulting polymer is not mentioned in any way, and the stability is unknown.
In Patent Document 4, a method for reducing coloring of a lactic acid-based polymer is described, wherein the polymer is subjected to heat treatment at a temperature at or above 120° C. and at or below the melting point under UV-irradiation in a nitrogen atmosphere or in the atmosphere, preferably in a nitrogen atmosphere. However, in Patent Document 4, although a coloring reduction effect is observed, no mention is made of residual catalyst. Moreover, this method, which requires an ultraviolet generator and also requires devising for effective irradiation of ultraviolet light, is applied to limited areas.
In Patent Document 5, a technique for obtaining an aliphatic polyester having excellent stability is described, wherein the polylactic acid obtained by a solid-phase polymerization method by using a volatile catalyst is subjected to heat treatment under gas circulation at a temperature at or above the reaction temperature of the solid-phase polymerization and below 170° C. In accordance with this method, it seems that the stability in forming and the storage stability of the aliphatic polyester are achieved by volatilizing or inactivating the catalyst comprising an organic sulfonic acid, but an applicable polymer requires use of a volatile catalyst, and is limited to those obtained by solid-phase polymerization. Furthermore, there is no mention of coloring.
As described above, conventional techniques related to purification of aliphatic polyesters such as lactic acid-based polymers focus on removal of the residual catalyst for the purpose of improving stability. No method is found for simultaneously reducing the coloring, residual monomer, and residual catalyst with simple equipment in an easy process. Moreover, use of a solvent and a specialized catalyst, or an ultraviolet generator requires a large amount of energy and is economically disadvantageous. Alternatively, in Patent Document 1, demonomerizing the resulting polymer is described, but no mention is made of coloring of the polymer, and the hue and the like of the resulting polymer are not known.